1. Field of the Invention
The present invention relates to a receiver for receiving angle modulated signals. Such a receiver can be a cordless or cellular telephone receiver, such as a DECT receiver, Digital European Cordless Telecommunications based on the ETSI DECT Standard, a PWT receiver, Personal Wireless Telecommunications based on the TR41.6 TIA Standard, a GSM receiver, Global System for Mobile Communications based on the ETSI GSM Standard, or any other receiver for angle modulated signals. In DECT, the angle modulated signals are GFSK signals, Gaussian Frequency Shift Keyed signal. In PWT, the angle modulated signals are π/4 DQPSK signals, Differential Quadrature Phase Shift Keyed Signals.
2. Description of the Related Art
In the handbook “Cordless Telecommunications in Europe”, W. H. Tuttlebee, Springer Verlag, 1990, pp. 211-214, a superheterodyne receiver architecture, and a direct conversion (zero IF) receiver architecture are disclosed, with a demodulator that is coupled to a noise limiting intermediate frequency filter. A high intermediate frequency is considered desirable to simplify the task of separating an image frequency from a wanted frequency in a frequency band. The choice of the intermediate frequency is also influenced by the technology available in relation to the modulation and signal bandwidth. In DECT, for instance, the signal band is in the order of 1 MHz. In receivers with so-called SAW filters, Surface Acoustic Wave filters, for DECT, usually the intermediate frequency is much higher than the signal bandwidth. Double superheterodyne architectures with two intermediate frequencies apply two stage IF filtering. In such an architecture, the first IF filter stage needs to have enough selectivity to provide adequate rejection of the image frequency of the second IF conversion stage. The direct conversion architecture is a one stage single conversion from RF to zero IF, the RF signal being mixed with in-phase and quadrature components of a local oscillator.
In other conventional digital communication receivers that use hard limited IF signals with FM discriminators, the IF frequency is normally much larger than the signal bandwidth in order to keep distortion in the demodulated signal small. For example, in DECT, which uses constant envelope GFSK modulation, hard limiting is usually done at an IF of about 10 MHz. In PWT, that uses π/4 DQPSK modulation, there is a similar requirement, with an additional need of a much larger linear range of the FM discriminator. Limiting is required in order to keep the input dynamic range of the demodulator small. It is much cheaper and easier to implement, as compared to applying automatic gain control (AGC). Another advantage of hard limiting is that the 1 bit quantized signal can be directly processed digitally, without the need of an A/D converter. Furthermore, it is advantageous to keep the IF frequency very low after the channel selectivity filtering. The demodulation can be fully integrated or even digitally done at lower sampling rates. But then, complicated distributed AGCs are neded to limit the dynamic range of the channel filters and the FM discriminator. In addition thereto, DC resetting techniques can be used to drastically reduce the DC offset created by LO leakage. Known zero-IF receivers for PWT with π/4 DQPSK modulation are even more complicated due to the non-constant envelope of the signal. Limiting cannot be used in zero IF receivers as the harmonics produced by them fold back into the wanted signal band. The effect is that the modulation phase is quantized into π/4, 3π/4, 5π/4, and 7π/4 by hard limiting, i.e., the phase quantization error is π/4. This is too coarse for achieving reasonable receiver sensitivity in terms of BER at a given Carrier to Noise ratio. Techniques have been proposed wherein four or more hard limited multi-phase zero IF signals are used to reduce the basic phase quantization error. The multiphase signals are generated using a linear combination of the basic non-limited I and Q quadrature signals. The phase quantization can be further reduced by using complicated architectures with high-speed counters that measure the intervals between zero crossings of the limited signals.